


Breeding Machine

by yastaghr



Series: Sanster Continuum [1]
Category: Undertale (Video Game)
Genre: Breeding, Consensual, Consensual Sex, Dom/sub Undertones, Eventual Relationships, M/M, Machine Sex, Much Comfort, Mutual Pining, Oblivious Sans, Other, Other Additional Tags to Be Added, Sex Magic, Smut, Soul Sex, Temporary hurt, cinnabunnies, oblivious gaster, scientists being dense
Language: English
Status: Completed
Published: 2017-08-05
Updated: 2017-08-05
Packaged: 2018-12-11 15:38:24
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,416
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/11717355
Author URL: https://archiveofourown.org/users/yastaghr/pseuds/yastaghr
Summary: The Royal Science team have been charged to remedy the Underground's population problems. Gaster creates a machine that should help. Sans isn't too sure.





	Breeding Machine

“Ah, Sans, just the monster I wanted to see. I think I’ve had a breakthrough on that seed transfer device we’ve been tasked with. This design worked in every simulation we’ve come up with. I’d love to hear your opinion.”

 

Sans turned away from his conversation with Alphys and ambled over to his friend and fellow scientist. 

 

“what’s it this time, g? a water balloon?”

 

The man chuckled and motioned him to follow, “Nothing so childish. Come on, I’ve got it set up in 3b.”

 

Hands in his pockets, he followed amiably. Puns were spouted and either ignored or choked at, depending on whether Gaster understood. The other remarked in passing at the closed doors of the operation suit, grumbling that if certain of his followers were having intercourse on the table  _ again _ he’d be royally pissed. This prompted more puns, which petered out just in time for Gaster to swing open the door to Testing Room 3b and wave Sans in. 

 

The amiable atmosphere evaporated instantly the moment Sans stepped in and saw it.

 

“gaster, what the heck are you thinking? nobody can handle this!”

 

Deep chuckles behind him did little to reassure Sans about Gaster’s sanity. For stars sakes,  _ he _ was the monster designated to test this thing! If the doc believed he’d be letting  _ that _ anywhere near his tiny, fragile body, he had another thing coming and it wasn’t praise.

 

“Really now, Sans, I don’t think it is as terrible as you’re making it out to me. Take a closer look, please.”

 

Eying his friend dubiously, he sidled closer. He had to hand it to Gaster, it certainly was pretty. Big, intimidating, and sure as heck not going inside him, but pretty. Marbled purple and white glass refracted the light oddly. Then again, the shape wasn’t exactly textbook for prisms. Phalluses weren’t, usually.

 

The thing was about a foot tall, base to tip. Just above the seat was an area of black rubber whose purpose was lost on Sans. Design, maybe? The base was worked through the chair. It looked like one of the smaller pumps leftover from Alphys’ experiment two months previous. Inflow came from some kind of black tank strapped to the bottom of the chair. The outflow tube snaked up the middle of the shaft to the slit. And wow, was that a weird slit. Much too wide, pointed at the center, and fitted with some kind of suction device. 

 

Wait, no, now he could place it. Damn, did G really think anyone was going to let him suction cup their SOULs? That was a laugh. It sounded really painful to him, but then again, he really didn’t know how this process was  _ supposed _ to work, either. That was the point. Too many monsters had died without passing on knowledge of their race’s reproductive process to their children. Now more than 25 of the 108 monster races in the Underground were threatened with extinction, and Asgore had asked the Royal Science Team to figure something out. And W.D. Gaster had come up with this.

 

“well, doc, i gotta hand it to you. this thing is absolutely the most terrifying thing you’ve ever made, and that includes the blasters. there is no way you’re getting me on this thing.”

 

Gaster looked at him with disappointment. His sockets started watering.

 

“no!”

 

The watering increased. Purple eyelashes batted at him.

 

“no! don’t you try that look on me, wingdings!”

 

Both hands came up in front of his chest. He clasped them, pouted his lip, and sank to his knees. Shit...he really wanted this, didn’t he? The eyes, the lip, the hands - he was even on his knees, and Sans  _ knew _ how much he hated that.

 

Sans’ left palm smacked into his forehead.

 

“fine. i’ll get on your stupid machine. but you better get some cinnamon bunnies, cause no way in hell is that not going to hurt me.”

 

Gaster rose to his feet with a wince. He brushed off his knees and smiled, pulling five steaming cinnamon bunnies out of his inventory and setting them on a table to one side. Sans scowled at them. Damn it, he’d been played. Again. This really was getting to be a problem.

 

“Thank you, Sans. Your sacrifice is brave and duly noted. I doubt we’ll need them, but I know they’re your favorite healing item, so I asked Papyrus to bring me some this morning. I let him know you might not be coming home tonight, so don’t worry.”

 

Sans scowled at Gaster this time, “thanks ever so much for your respect of my independant agency. when this goes wrong, just know that my last, dying breath will be spent cursing your name.”

 

Gaster nodded and pulled out a purple tube of gel, “Of course. When this goes well, I expect fresh coffee for a week. Do you need a hand?”

 

Sans kicked his shorts off with a shiver. They landed in a rough heap with the rest of his clothes, removed quickly during the familiar speech.

 

“no thanks. i take it i’m supposed to sit on this thing?”

 

Gaster unscrewed the lid and squirted half the tube onto the glass dick.

 

“Yes. No genitals, just thread it through your pelvic inlet. I’ll help you get settled.”

 

Sans hesitated next to the chair. 

 

“you’re sure it’s going to fit in there, doc? i’m pretty small.”

 

Gaster scooped him up under both shoulders and lifted him in the air. Unlike every other time someone taller (i.e. everybody) picked him up, Sans wasn’t so sure he wanted to be put down.

 

“What do you take me for, Undyne? I measured three times before we built those simulations, and I measured twice again before I finished the mold. It’s going to fit. In fact, there should be about two centimeters of leeway. Does that make you feel better?”

 

Sans sighed, telling his racing SOUL to quit it.

 

“yeah. put me on that thing before i regret it.”

 

“Gladly.”

 

One thing the doc hadn’t mentioned was it was COLD. Sans nearly shivered himself into a fit when the lubricant hit his pubic symphysis. Gaster had to stop moving entirely until Sans got himself together enough to quit panicking. Finally, though, Sans was being lowered.

 

His first thought was that it was weird. Really, really weird. Logically, given that nothing but lubricant was touching him, he shouldn’t be able to feel anything. He’d dropped marbles and even golf balls down through his pelvic inlet as a kid. Hadn’t felt a thing. But now this...his mind couldn’t focus on anything but the feeling of invasion, of something alien pushing into him and occupying  _ his _ space. 

 

The entire volume of it was creepy, slimy cold radiating off it. Not really, but that’s the only way he could think to describe it. There was also another thing, buried under that, that he’d almost call...pleasant? A faint magical signature was embedded in the thing, not enough for him to identify it, but there all the same. It made it feel safer, like when Papyrus held his hand while Grillby set a broken limb. It was like he trusted it. It felt like it cared about him, loved him almost, and that getting close like this was it’s way of showing that. It felt, well, pleasant. 

 

He started when the varnished wood clanked beneath him. Was that it? It didn’t feel nearly as big as it had looked to him earlier. Then Sans tried to turn and it pressed against him. Every vertebrae from coccyx to T8 was touched by it.  That...that was a lot.

 

The hands cupping his humeri retreated. Gaster came into his field of vision. His hands were wiping off the pale goo onto his lab coat, leaving yet another stain. He pulled a pen out of his breast pocket as he sat. He grabbed a clipboard off of the table and scribbled in the margins experimentally. Apparently satisfied with the flow of ink, he turned his head back to Sans.

 

“Alright. I’d like to get your first impressions before we finish hooking you up. How does it feel, Sans?”

 

“well, doc, you’ve officially filled your creepiness quota for the year. this thing’s cold and...weird!”

 

Gaster gave him the stink eye, “You have a PhD, Sans. I expect better words out of you than ‘weird’.”

 

Sans shifted slightly, and the rod inside him slid across the inside of his ribs. He hissed, and tried to put words around the sensation. It was hard, given the way his attention kept focusing on the shape within him.

 

“it’s...not what i expected. having something sitting inside me is occupying more of my attention than i’d thought. i wouldn’t want to try working on anything delicate or complicated with this in me, that’s for sure. it also kinda...radiates? it feels like accidentally walking through one of the blook family. especially if they’ve just been at the snails - there’s a lot of slime. not that i’m objecting, but it’s slime. not everyone feels the same way.”

 

Gaster scribbled furiously, and Sans resigned himself to transcribing those notes later. He should have asked G to use one of the tape recorders. At least then it didn’t give him a headache.

 

Sans let his focus drift up to the ceiling as his thoughts wandered, “there’s also an undercurrent of pleasantness? don’t know how you did it doc, but it feels like this thing cares about me. it helps mitigate some of the creepiness, anyway. also i’m getting shivers anytime it touches me. kinda tingly under the slime and the cold. not sure about those.”

 

He thought for a while, wiggled around a bit to see if any new sensations were forthcoming, then shrugged.

 

“that’s about it. weird, slimy, and cold - but not unpleasant. maybe heat it up next time? or warm up the lubricant, at least.”

 

Gaster’s clipboard settled onto the table, “That’s good. I was hoping the alien sensations wouldn’t be quite that strong. The simulation mentioned something of the sort was a possibility. In any case, a warmer introduction is probably a good idea. It hadn’t occurred to me that the temperature difference would be that jarring. A note for the future, certainly.”

 

Gaster now crouched in front of him. For once his friend’s sockets were on a level with Sans’.

 

“I’m going to finish the attachment now. Okay, Sans?”

 

The word “attachment” pierced through his fascination with the intrusion in his pelvis and sparked an objection.

 

“sorry, g, but i  _ really _ don’t think i’m able summon my soul like this.”

 

Gaster smiled mischievously back at him, “That’s alright. I didn’t think you would be.”

 

His hand snaked down and flipped a switch. A rush of liquid filled the room. Sans stared at Gaster, who smiled back. He opened his mouth to ask what the heck was supposed to-

 

“hnrgh!”

 

Sans arms shot out and held onto Gaster as the thing inflated. The section of rubber just above the base had ended up resting right above the bottom curve of his pelvis. The switch had triggered some kind of inflation, and now the stiff balloon had bulged out. It pressed against his pubic symphysis and was still expanding. Within a minute the entire bowl of his pelvis, his sacrum, and his coccyx were all being pressed into. 

 

A click below him and the inflation ended. Sans was panting hard into his friend’s shoulder. Any coherent thought he had had was quickly vanishing.

 

“From your reaction, I’d say the artificial knotting system had the desired effect. I was worried the reaction wouldn’t cross racial boundaries, but the canines assured me that it would. I’ll have to thank them. A crate of dog treats perhaps?”

 

Gaster hummed thoughtfully as Sans tried to pull himself together. This felt weird. Not unpleasant, but the sensation was so unlike anything he’d ever felt before that he had no words to describe it. He tried to stop panting and ended up letting out a dramatic whine. His phalanges dug into Gaster’s sweater desperately.

 

Gaster rubbed his back soothingly, “Shh. Shh. Don’t worry, Sans, I’m here. I’m not going anywhere while you’re on this. It’s okay.”

 

The pressure was overwhelming him. He tried to shift around, but every movement he made just made it worse. Sans couldn’t think about anything else now. His entire mind was just trying to handle all the sensations coming in from that device. The hand on his back only made things worse, pressing his spine up against the shaft and sliding him around. 

 

Something in him gave, and Sans started crying in earnest. It wasn’t dangerous, but the weird feeling inside him was almost painful. He wanted it to stop, go away, slow down - anything but drag on like this. He wanted to be done.

 

The moment that thought crystallized, Sans felt his SOUL condense. Unlike normal, it felt...wet. That same sliminess from earlier was dripping off him and he didn’t know why. The crying just got worse. He was embarrassed, he didn’t want Gaster to see, he didn’t-

 

Gaster’s hand gently cupped the bells of his SOUL. Wisps of emotion radiated off his friend - fascination, gratitude, triumph, affection, and curiosity were uppermost. Figures - Gaster was always curious about everything. Even his pitiful little SOUL wasn’t safe from that.

 

“Thank you so much, Sans. This is the last step before I’ll have finished hooking you up to the machine. It might hurt a little bit since you’re so tense, so I’m going to grab one of the cinnamon bunnies and put it in your mouth. If you feel anything at all, just bite down. Okay?”

 

Sans nodded violently. Most of him just wanted to get this over with, and even a little pain would be better than this waiting. 

 

Another part, small but growing in volume, was telling him it would all be worth it to see Gaster happy with him. He’d been so downhearted these past few weeks. Design after design had failed the tests before it could even be built. And for someone still coming down from the high of completing the CORE, that kind of disappointment hurt. If Sans could just make it through this, Gaster would once again have made a winning design. He’d be so happy, and Sans would have been the cause of that. That little part of him didn’t care about the pain; pain was nothing to that kind of happiness.

 

“Open, please.”

 

Sans had missed the transition, but with the smell of fresh baked cinnamon bunnies in his nasal cavity, he couldn’t help but comply. Gaster balanced the treat on his tongue easily. After wiping the sugar off onto his coat, he reached down and slipped the second hand underneath Sans’ SOUL.

 

“Good. Now, I’m going to lower you onto the suctioning device. Let me know if it pinches you, we want a smooth seal for the next bit.”

 

If he had thought the slimy sensations were bad before, they were nothing compared to what it felt like when that goop touched his SOUL. It felt  _ wrong,  _ detached, uncaring, isolatingly cold, and alien. Sans yelped.

 

Gaster’s hands yanked his SOUL away and cradled it against his chest, “Sans?!”

 

Words washed over Sans unnoticed as something else swam into his awareness. Magic familiar as his own was right there, right next to him, but...twice? It was a foghorn...and also barely a whisper. And separate? How-

 

Frantic phalanges scraped at his SOUL. The goop was gone, banished. Sans could think again...mostly. The scrape of contact had at least brought him out of that daze. He still had a giant dick inside him, but hey, at least he wasn’t being accidentally severed from his own SOUL.

 

“g?” 

 

His voice quavered. Gaster flung himself at the chair. His whole body wrapped around Sans. If it weren’t for the rod still inside him and the sensation of his own ribs and Gaster squeezing his SOUL, Sans would have found it pleasant. It was still nice, just...less enjoyable. It was infinitely better than it had felt being cut off like that, though.

 

After a minute, his friend’s words finally stopped being a pudding of sobbing and started taking shape. A very repetitive shape.

 

“-sorry I’m sorry I’m so sorry Sans I-”

 

His SOUL was only inches away from both Gaster and the artificial dick inside him. It decided that now was a good time to tell him that the familiar magic was close in both ways now. Without the goop interfering, that suddenly made a lot more sense.

 

Oh. OH.

 

“g.”

 

The scientist snuffled loudly. He’d always been a messy crier, and Sans was less embarrassed to know that now than he was five minutes ago. Being surrounded inside and out by someone who clearly had a thing for you does that.

 

“g, i don’t think the cream was a good idea.”

 

A slightly soppy Gaster questioned him silently.

 

“you put the barrier cream alphys made last week into the goop, right? to mask your signature?”

 

Guilt, fear, and shame battled over the soggy face.

 

“yeah. thought so. it’s strong stuff, doc. like, undyne-giving-you-a-suplex strong.” 

 

That didn’t seem to be sinking in. Sans tried again.

 

“like, severing-your-soul-from-your-body strong, g.”

 

That one worked. Panic was definitely an informed and logical response here, obviously. No emotional entanglements there, right enough.

 

“you got it all off the first time, doc. now it just hurts.”

 

The scraping at his SOUL stopped abruptly.

 

“Oh no, I- Sans, I didn’t mean to-”

 

“ **gaster** ,” his friend focused on him, “it’s fine. just...scrape the goo of your machine. please? i’d rather feel you anyway, and having my soul severed was definitely not on my list of activities for today.”

 

Gaster sniffed and bent down. It took several seconds for Sans to register the sounds he was making, which was kind of embarrassing. He’d been a scientist for years - he damn well knew what scraping glass sounded like, and this wasn’t it. 

 

“g, what are you doing?”

 

Gaster started. Something bumped into the edge of the chair, and Sans shuddered as the entire setup rocked. The pressure from the artificial knot rolled around his pelvis in a very distracting way.

 

“I’m...turning it off?”

 

“why?”

 

“Because you asked me to?”

 

Gaster didn’t sound certain. Sans sighed and reached out a hand demandingly, just as Papyrus would to anyone within range. Gaster took it automatically, and Sans squeezed.

 

“gast, i still wanna do this. just take off the goo, thanks.”

 

Gaster gulped. His sockets flitted across Sans’ face. The seated skeleton just radiated as much confidence and warmth as he could with the goo-coated machine still in him.

 

The other hesitantly reached up into Sans’ ribcage and started scraping. The motions made the glass dick shiver slightly, and that translated into the membrane of the knot rather well. Sans found himself moaning at the sensation. It wasn’t anywhere near as overpowering as the inflation had been, and like this it felt really, really good.

 

Gaster paused at the first moan. Then a glimmer of his usual humor worked its way through the massive guilt. He kept scraping, switching up his rhythm and intensity to make Sans gasp. The bones currently laced with his starting squeezing, and he soothed them with his thumb.

 

Sans chuckled weakly, “this feels great, doc. sends shivers up my spine.”

 

Gaster lent around to examine the bones in question. He slid his gloved phalanges down every single vertebrae the dick had touched. Every new ridge cleared made Sans shiver more, as more and more of the goo was pried off his bones. He hadn’t realised how much it was blocking from him. This was getting intense.

 

“Should I get a towel?” Sans panted up at him, eye lights losing focus. Gaster laughed, “Having a hard time there, Sans? You’re drooling.”

 

“fuck you this feels amazing.”

 

Gaster laughed, “Oh Sans you are adorab- I mean-”

 

Sans circled two phalanges around the carpal bones of the hand in his grasp.

 

“don’t -hah- hold back -hah- on my ac- hah -ount, doc.”

 

Gaster snorted, flinging a large globule of goo off into a distant corner.

 

“Of course not. It’s not as though your ego can get any bigger - I heard your brother talking about getting you another bed just to fit it.”

 

Sans scowled at him. That wasn’t what he meant. Gaster could be absolutely brilliant in some ways and dense as iron in others.

 

The skeleton in the chair pushed his skull up to meet Gaster’s and huffed out magic in a huge lungful. It was as close as he could get to a kiss, and right now, probably more intoxicating.

 

Gaster blinked slowly, his eye lights burst into short, wide hearts. Sans smiled at him and sent a softer, more carefully crafted wave of magic his way.

 

“i like you too, bonehead.”

 

“Oh.”

 

Gaster looked stunned. Sans snorted.

 

“oh is right. now fire up this little machine of yours before I really get annoyed.”

 

Gaster blinked out of his stupor, “Really?”

 

Sans rolled his eye lights, “yes. machine?”

 

Gaster stared at him for several long seconds. Then he hopped, fell to his knees, groaned, and recollected himself.

 

“Oh. Yes. Right. Moving on.”

 

He ran one frantic glance over the entire array, checking it for any remnants of the goop.

 

“Right. Where did I put your SOUL?”

 

Sans snorted, “it’s in your hand, g. not that one, the other one. no, try again. aah- yes. that one.”

 

Gaster smirked and Sans’ squeal, but chose to move the hand in question over to the machine and start attaching it. This time went much better than the last. The suction cup felt weird, like being wrapped in a big plastic burrito. Gaster’s cautious strokes made it fell all the better.

 

Finally, it was in and sealed to Gaster’s satisfaction. He moved to Sans’ front with a sheepish look. 

 

“The next part is, uh, most key to this process. When I turn on the machine it will pump seminal magic up the interior tube. It exits at the crux of your SOUL, which should be fixed in place. Can I, um, hold it for extra certainty? Oh. Okay. Good.”

 

Sans stopped buffeting him with magic and made a ‘go on with it’ gesture.

 

“I’m not certain of the science of what happens next. From what the Blooks told me the magic shouldn’t be absorbed but, uh, surrounded. There’s some kind of cavity, and it should fill that and then you should, um, seal it up. It’s vitally important that you don’t absorb it, they said. That wouldn’t do what we need.”

 

Sans took all this in with as much seriousness as he could muster. One detail stood out.

 

“seminal magic? can i ask who’s the donor or is that not allowed?” Gaster’s look of embarrassment told him everything he needed to know, “kay. fire when ready, oh bearer of loins.”

 

Gaster must have pressed some button in his coughing fit, because one moment Sans was smirking down at him, the next the entire device was rattling fiercely. Sans bit back a cry because it felt amazing. He really didn’t want Gaster to turn it off again.

 

A thin stream of air pushed against the base of Sans’ SOUL. It passed through easily enough. Intentionless air had no effect on magical organs. As it made its way out, Gaster managed to collect himself enough to brace Sans’ SOUL from above, which was just as well.

 

Having magical fluid press against your SOUL isn’t the most pleasant of processes. Sans had to fight down the urge to absorb it as he would medicine or ketchup. But within seconds he felt something in his SOUL changing. It wasn’t much, just a tight slit parting the center to form a firm, natural cavity. It didn’t hurt, which Sans had been afraid of. It actually felt...good?

 

Really good, actually. The bit of him that had so deeply wanted to please Gaster was screaming that this was the way, and who was Sans to argue with something like that? Especially when the liquid parted the outer lips and spurted inside.

 

“hanh!”

 

Gaster’s thumbs stroked evenly around the edges of his SOUL.

 

“Easy, Sans. I’ve got you.”

 

Sans trembled with the force of that liquid. It felt viscous, almost Newtonian within him. But having it pressing on every inch of that newly formed wall made him want to scream with pleasure. This was amazing! Why hadn’t anybody told them about this? He could feel, just barely under the weight of all that pleasure, that Gaster himself was enjoying this too. Yes!

 

Something burst inside him that had Sans shivering and blubbering incoherent fragments of Gaster’s name. The thumb kept stroking all the while, and after a few seconds that felt like a lifetime he could make out Gaster’s voice.

 

“-ell me when you can understand this. The Blooks mentioned you might ‘lose it’ for a bit, but I would very much like to know you are okay.”

 

“m’fine, doc. great.”

 

The words were a croak, but the response was immediate. Conjured hands soothed every heated inch of him while the ones on his SOUL gently disconnected it.

 

“May I examine your SOUL, Sans?”

 

Sans nodded, winded. Gaster ran sensitive phalanges along the line going down from tip to crux. It sent pleasant shivers down Sans’ spine, but the area surrounding the merge of both bells was extremely sensitive. Sans yelped.

 

“Sorry,” Gaster let go immediately, running a quick CHECK on him, “I shouldn’t have done that. You don’t look like you lost any HP, but better safe than dead.”

 

He creaked up from his kneeling position and wobbled over to the table. The last cinnabunny was picked up and served to Sans. The skeleton, prompted by a curious instinct, wolfed it down. The magic settled firmly into the curves of his bells and...reinforced them? That’s what it felt like, anyway. 

 

Gaster’s cautious hands confirmed this a moment later. Sans’ SOUL was apparently bulging at the moment. Careful probes showed that it had developed a double layer at the bottom - the innermost swollen and parted to reveal a hint of purple, the outermost solid as the tip proved to be. 

 

“well, that’s a definite result.”

 

“Yes. It will be interesting to watch as this-”

 

“g?”

 

The brilliant scientist looked down at his colleague, “Yes?”

 

“get me off this thing.”

 

“Right.”


End file.
